The present invention relates to security and authentication and, in particular, it concerns polymer layers having latent images visible in polarized light and methods for generating such layers.
It is known to produce various copy-resistant features in manufactured products to guard against forgery. Typical examples of such features are water-marks, holograms, micro-printing and embedded metallic strips.
An alternative type of protection could be provided by features which, although not immediately visible, may readily be checked to confirm the authenticity of a product. One attempt at such a feature employs implanted liquid crystal optical elements which are configured to affect the polarization of transmitted light such that, when viewed through a polarizer, an identifiable pattern is seen.
Examples of such a structure are described in European Patent Publications EP 689 065 A1 and EP 689 084 A1. The structures disclosed are multi-layer structures forming optical components based on a photo-crosslinked liquid crystal "monomer". Formation of the optical components in the liquid crystal polymers requires multiple coating of the substrate material with subsequent photochemical, electrical, magnetic and mechanical processing.
The structures described in the aforementioned documents suffer from considerable practical limitations. Firstly, formation of multi-layer liquid crystal components with separate processing of each layer requires complex and expensive manufacturing equipment and materials. Secondly, the various layers making up the liquid crystal components differ in their optical, chemical and mechanical properties. These differences may lead to de-lamination and/or image quality deterioration when the structure is exposed to thermal, chemical or mechanical stresses. Furthermore, the complexity of the multi-layer structure rules out its use in certain applications such as, for example, flexible film. It is also important to note that liquid crystal images are normally visible when viewed directly by the human eye.
The theoretical background of the formation of stable latent images (SLI) in polymers was described in SPIE Volume 2722 (page 252, Smart Structures and Materials, 1996): "New photochromic materials for visible range" by A. Vannikov and A. Karassev, which is hereby incorporated by reference in its entirety as if set out herein. This paper describes the scientific principles of SLI formation in polymers, but deliberately withheld and misrepresented certain information to ensure that no enabling disclosure was given. Specifically, the paper omits accurate details of three crucial processing parameters:
1. The chemical composition that creates photosensitivity in the polymer matrix. The correct concentration of the elements forming the composition is critical. Wrong concentration causes formation of visible image or no image at all instead of SLI. PA1 2. The chemical composition of the developer/fixer. Here again, SLI is extremely sensitive to the chemistry of the composition. PA1 3. The order of the development/fixation process
There is therefore a need for latent image products in which a latent image, visible in polarized light, is provided within a single polymer layer, which are convenient and economical to produce, and which are structurally simple and mechanically stable such that they can be used in a wide range of practical applications. It would also be advantageous to provide straightforward and economical methods for producing such products.